Many electronic devices require a direct current (DC) voltage power supply, such as light-emitting diodes (LEDs). Generally, switch mode power supplies are employed to provide the DC voltage in different applications. In most applications of switch mode power supply, a transformer or an inductor is employed as an energy storage component. For example, a transformer is employed as the energy storage element in a flyback converter, and a switch of the flyback converter is coupled to a primary winding of the transformer. A control circuit is employed to turn ON or turn OFF the switch, the transformer is configured to store energy or output energy to a secondary winding of the transformer alternately, and then the light-emitting diodes are driven.
Fixed frequency control and varied frequency control are two main control strategies for switch mode power supply. Quasi-resonant (QR) control is widely used as one of the varied frequency control. FIG. 1 shows waveforms illustrating signals of traditional switch mode power supply with quasi-resonant control. As shown in FIG. 1, the switch mode power supply works in boundary conduction mode, and an energy storage component is resonant with a parasitic capacitance of a switch when a current IS flowing through the energy storage component is decreased to about zero. The switch is turned ON when a drain-source voltage VDS across the switch is almost at its minimum value (i.e., valley turn-on), as a result the switch loss is small. The switch is turned OFF to regulate an output of the switch mode power supply when a current flowing through the switch is increased larger than a feedback signal indicating output voltage/output current/output power.
Traditionally, the switch mode power supply comprises an integrated circuit (IC) as its control circuit. The control IC comprises a first pin coupled to a current sampling signal indicating the current flowing through the switch and a second pin coupled to a voltage sampling signal indicating a voltage across the switch. The control IC is configured to turn OFF the switch when the current sampling signal is larger than the feedback signal and the control IC is configured to turn ON the switch when the voltage sampling signal is less than a threshold signal.
The control IC may have a cycle by cycle current limit to avoid an over-current breakdown of the switch. The current sampling signal is compared with an over-current threshold during each switching period. When the current sampling signal is larger than the over-current threshold signal, the control IC is configured to provide an over-current protection, e.g., turning OFF the switch until the control IC restarts.
For traditional switch mode power supply, the over-current threshold signal is integrated onto the control IC and is non-adjustable, while the current sampling signal has to be constant per an anticipated output voltage/current/power of the switch mode power supply. Thus, a switch mode power supply with a flexible current protection is needed.
The control IC may comprise a leading edge blanking (LEB) unit to avoid turning OFF the switch by mistake when the switch is turned ON and the parasitic capacitance of the switch is discharging rapidly. During a LEB period, e.g., 200-300 ns, the current sampling signal is not effect for protecting. But the switch may be broken down because of a non-detection of a potential over-current during the LEB period.